Boiler plants, particularly those having coal-fired boilers, are well-known sources of pollution. Sulfur emissions from these plants has become a matter of heightened concern in recent years, in light of the perceived effects of "acid rain." Although pollutant emission reduction has long been the object of scientific inquiry, there is still a great demand for methods and apparatus that are effective, easily retrofit onto existing plants, and which convert the collected pollutants into useful and valuable byproducts.
The prior art recognizes that cooling flue gases that contain sulfur trioxide and water vapor will result in condensation of sulfuric acid. See U.S. Pat. No. 4,526,112 to Warner; U.S. Pat. No. 4,874,585 to Johnson, et al.; U.S. Pat. No. 4,910,011 to Dorr, et al; and Bovier, Sulfur-Smoke Removal System, 26 Proc. Am. Power Conf. 138 (1964). Other systems of the prior art employ high-volume sprays of water or other liquid agents to scrub particulate matter and pollutants from the flue gas. See U.S. Pat. No. 3,442,232 to White; U.S. Pat. No. 3,456,928 to Selway; and U.S. Pat. No. 3,770,385 to Grey et al.
It has further been recognized that when a tube-type heat exchanger is located in the path of a flue gas flow, the particulate material in the gas can collect on the heat exchanger tubes. Cleansing sprays have been provided in accompaniment with the heat exchangers according to some prior art methods, in order to wash the collected particulates from the heat exchangers. See U.S. Pat. No. 4,452,620 to Dosmond; and U.S. Pat. No. 4,526,112 to Warner.
Although all of the aforementioned systems are more or less effective in removing some degree of the pollutants from the flue gas, they merely shift the environmental burden from atmospheric waste to surface waste. Attempts have therefore been made to convert pollutants into useful end products as they are removed from flue gases. One such approach involves the introduction of ammonia to combine with sulfur oxides in the flue gas and form ammonium bisulfite or ammonium bisulfate. See Studies on the Recovery of Sulfur Dioxide from Hot Flue Gases to Control Air Pollution, R. Kiyoura, et al., 81 Bull. Tokyo Inst. of Tech. 1 (1967); Slack et al., Sulfur Dioxide Removal From Waste Gases 176 (1975). The combination of ammonia and sulfur oxides according to these references produced a dry ammonium bisulfite or ammonium bisulfate product. Neither of these systems has been commercially adopted, however, due to drawbacks that have until now remained unsolved. Chief among these drawbacks is the impediment to gas flow which the solid precipitate causes, especially where closely-packed heat exchanger tube arrangements are employed. In addition, the full potential of this approach for substantially complete removal of pollutants including sulfur and nitrogen from flue gas has not been realized.
It is therefore an object of the present invention to provide a system for removing pollutants from flue gas by which substantially all oxides of sulfur and nitrogen are removed.
It is a further object of the invention to provide such a system in which the harmful pollutants are not merely removed from the flue gas to be disposed elsewhere, but are converted into useful and environmentally safe substances, particularly ammonium bisulfite and ammonium bisulfate.
It is another object of the invention to provide such a system in which the formation of useful byproducts does not cause obstruction of the gas flow or hamper the proper functioning of the apparatus.
It is yet another object of the invention to provide such a system which is readily retrofit onto existing boiler plant exhaust equipment.